Function spike_train_pca 

pub fn spike_train_pca(
    trains: &[&[i32]],
    n_components: usize,
    bin_size: usize,
) -> (Vec<f64>, Vec<f64>)

PCA on binned spike count matrix (neurons x time_bins).

trains: list of binary trains (each &[i32]). Returns (projected, explained_variance_ratio) where projected is row-major (n_components, min_bins).